This invention relates to brazing and soldering processes and, more particularly, to the formation of vertically oriented brazements in low-wetting materials, particularly in single crystal silicon.
Brazing is a method of joining metals and other materials though application of heat and a brazing filler metal to such metals and other materials. The filler metal is dissimilar to the materials being joined together in the brazement and possesses a melting temperature above 840 degrees F., but below the melting temperature of the metals or materials being joined. As example, with the filler positioned between the two surfaces to be joined, the temperature is raised to the melting temperature of the filler, the brazing temperature, but below the melting temperature of the material of the joining surfaces. The filler becomes molten, wets the adjacent surfaces of the joining materials, and fills the gap between those adjacent surfaces. As another example, braze material is placed at the entrance to the joint, the temperature is raised to the melting temperature of the filler, the brazing temperature, but below the melting temperature of the material of the joining surfaces. The filler becomes molten, wets the adjacent surfaces of the joining materials, and, through capillary action, is drawn in to the gap between those adjacent surfaces. In either procedure, when cooled, the brazing material re-solidifies and forms a strong metallurgical bond with the joined materials at the juncture, the brazement or braze joint. That bonding occurs at the molecular level. A properly constructed braze joint provides a high degree of serviceability under concentrated stress, vibration, and temperature loads.
Soldering is similar to brazing. The difference is that the filler material for soldering (eg. the solder) is a metal or metal alloy that has a lower melting temperature than brazing material. Typically, the solder is not as strong as brazing material and does not produce a bond that is as strong as that produced by brazing material. The present invention is directed to brazing and soldering processes. Hence, it should be understood that when reference is made herein to brazing, the term is intended to also encompass and include soldering, even if explicit reference is not made to solder or soldering.
Known brazing materials include pure metals, such as copper, gold and silver and complex alloys of aluminum, gold, nickel, magnesium, cobalt, silver and palladium, as example. No single one of those filler materials is of universal application in brazing.
The two basic braze joint designs are the lap joint and the butt joint. In the lap joint a portion of one surface overlaps a portion of another surface, and the brazement is formed in the region of overlap. In the butt joint, the two materials to be joined contain end surfaces that are butted against one another. The present invention is applicable to the formation of both such braze joints. More particularly, the invention is applicable to joining two surface regions of like shape and size in which the joint or bond line between the two surfaces is accessible for wrapping in the manner later herein described.
In certain circumstances the brazing of a butt joint is performed with the juncture between the materials to be joined being vertically oriented. In that situation, the capillary in-draw of the molten brazing or soldering metal in-between the close fitting butted surfaces of the parts to be joined typically occurs with inconsequential melting of the joined materials and, when the materials are cooled down and the filler solidifies, a strong vertically oriented braze or solder joint is formed between those parts.
Capillary in-draw serves as a good indicator of the ability of a braze material to wet the materials that are to be brazed together. If for a given joint material, it is found that the molten braze material is not drawn into the gap between the close fitting surfaces of the joint in more than an insignificant amount, the joint material is said to be xe2x80x9clow-wettingxe2x80x9d in the context of the present invention. If the materials to be brazed together are low-wetting, then a braze joint cannot be formed in the foregoing process. As example, silicon is a material that is low-wetting. As an advantage, the present invention accomplishes satisfactory brazing of low-wetting materials, such as silicon.
A flux is sometimes used in the brazing or soldering process to clean the surfaces of the joining material of oxides and or other impurities that inhibit the wetting of the surfaces of the joining material by the molten filler. Other brazing and soldering processes do not incorporate the application of a flux to the joining materials and is called xe2x80x9cfluxlessxe2x80x9d brazing (or soldering). For reasons not here material, the chemical ingredients that function as the flux are typically undesirable and are not used so as to avoid chemical damage to the joining surfaces or to avoid environmental issues. The present invention also has application to fluxless brazing or soldering.
Fluxless brazing of silicon (and other materials that are either chemically reactive with the filler or possess a non-wettable surface) present special challenges. The surface oxide that naturally forms on the silicon crystal inhibits the molten filler from wetting of the crystal surfaces and, hence, inhibits capillary flow of the molten filler, while penetration of the molten filler through cracks that inherently form in the surface oxide of the silicon crystal results in eutectic melting. As X-ray inspection and microscopic examination reveals, the result is that capillary retention of the brazing or soldering material in the bond line is poor, and bond line porosity is higher than desired. Although a braze joint is formed in the silicon crystal that is useful for some applications, the joint lacks the uniformity and strength common to braze joints produced in metals and is regarded as being of low quality. The present invention provides the method and means to improve the quality of a vertically oriented braze joint produced in silicon and like materials.
Accordingly, a principal object of the present invention is to provide a method of brazing of low-wetting materials.
Another object of the invention is to produce high quality brazing and soldering of single crystal silicon material.
A further object of the present invention is the production of high quality vertically oriented braze and solder joints in materials having low-wetting characteristics.
The foregoing objects and advantages are realized by a new method of brazing of pieces of low wetting materials, such as single crystal silicon, that is accomplished by sandwiching the brazing material between the pieces and sealing the edges of the brazing material with a tape wrapped about the periphery of the brazing material, mechanically placing that tape in compression and maintaining that compression while heating the sandwiched assembly to the brazing temperature of the brazing material. The tape possesses the characteristic of being non-wettable or non-wetting by the brazing material in the molten state and, hence, is impermeable to that molten material. In accordance with a more specific aspect to the invention, the foregoing tape may be of a fibrous porous ceramic material.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.